Electron-beam furnace with opposedfield magnetic beam guidance



Apnl 30, 1963 H. w. HOWE 3,087,211

ELECTRON-BEAM FURNACE WITH OPPOSED-FIELD MAGNETIC BEAM GUIDANCE FiledMay 27, 1960 '2 Sheets-Sheet 1 W P 4 Xf/mm April 30, 1963 H. w. HOWE3,087,211

ELECTRON-BEAM FURNACE WITH OPPOSED-FIELD MAGNETIC BEAM GUIDANCE '2Sheets-Sheet 2 Filed May 27, 1960 INVENTOR. HOAMPD 4 How: BY

P424, fif/wzdukwz fiTTUF/V! 3,087,211 Patented Apr. 30, 1963 3,687,211ELECTRON-BEAM FURNACE WITH OPPOSED- FIELD MAGNETIC BEAM GUEDANCE HowardW. Howe, Oakland, Caiifi, assignor to Stauifer Chemical Company, NewYork, N.Y., a corporation of Delaware Filed May 27, 1960, Ser. No.32,216 4 Claims. (Cl. 22--57.2)

This invention relates to electron-beam furnaces for heating materialsby electron bombardment in a high vacuum, and particularly for meltingmaterials and casing ingots therefrom, with resulting purification,degasifica tion to an exceptionally high degree, and other benefits. Anobject of the present invention is to provide electronbeam furnacescapable of larger scale and more economical operations. Other objectsand advantages will appear as the description proceeds.

Briefly stated, electron-beam melting and casting furnaces include,within a continuously evacuated tank, a container for the moltenmaterial, which most commonly has the form of an annular, water-cooledcasting mold open at its top and bottom ends. Solidified material may beprogressively withdrawn through the bottom end of the mold to form acast ingot of progressively increasing length. An electron gun directs abeam of electrons downward into the open top end of the mold, to bombardthe material therein and maintain a pool of molten rnaterial atop thesolidifying ingot. Melt stock is progressively fed into the beam, eitherhorizontally from the side or vertically from above, and the melt stockis continually melted off as it advances into the electron beam. Theso-melted material falls into the open top end of the mold forcontinually replenishing the molten material in the pool.

Generally speaking, it is more desirable to feed the melt stock inhorizontally from one side of the beam during the initial processing ofmaterials containing considerable amounts of volatile impurities,because the horizontal feed-in usually results in a more openconfiguration facilitating the evacuation of evolved gaseous matter. Butfor remelting previously cast ingots, e.g., for further purification orother improvement of ingot quality, vertical feed-in is preferred, amongother reasons, because the more closed configuration, particularly theproximity of the melting surface of the melt stock to the molten pool ontop of the newly cast ingot, with the two heated surfaces substantiallyfacing each other, conserves heat and considerably reduces theelectrical power consumption.

The copending applications of Hugh R. Smith, Jr., Serial No. 32,215,filed May 27, 1960, entitled Electron- Beam Furnace with MagneticallyGuided Beam and of Charles W. Hanks, Serial No. 32,217, filed May 27,1960, entitled, Electron-Beam Furnace with Double-Coil MagneticGuidance, both assigned to the same assignee as the present application,disclose and claim improved electronbeam furnaces utilizing magneticfields to guide the electron beam into the open top end of the castingmold.

The present invention is a further improvement, which provides furtherbenefits and advantages, including the following: the electron beam canbe introduced at a much smaller angle to the horizontal, and thereforecan travel inward much farther, e.g., between the molten pool within theupper part of the casting mold and the melting bottom end of a verticalrod of melt stock closely spaced above the melting pool in coaxialalignment with the mold. This makes possible the remelting and recastingof larger-diameter ingots, with the advantageous vertical feed-in of themelt stock, and appears to overcome all of the limitations heretoforeexisting on the maximum diameters of ingots that could be processedsatisfactorily. Also, annular electron guns of much larger diameter cannow be employed. This increases the spacing between the gun and sourcesof evolved gaseous matter, thereby increasing gun life, and also makespossible significant increases in the total beam current in relation tothe maximum intensity of the electronic current in the immediatevicinity of the electrodes of the gun.

In accordance with the present invention, two vertically spaced, annularelectromagnet windings are employed, one extending around the annularmold and the other vertically spaced above the open top end of the mold.The two windings are in vertical, coaxial alignment and are energized inbucking magnetic flux relation. As a general rule, the ampere-turnsenergization of the lower Winding exceeds that of the upper winding. Theso-energized windings produce a magnetic field having certain flux linesthat converge inwardly and downwardly between the two vertically spacedwindings and into the open top end of the mold. These particular fluxlines extend through and are primarily generated by the lower winding.The bucking field produced by the upper winding causes the flux linesunder consideration to pass between the two windings and approach thecasting mold at a smaller angle to the horizontal, which can be adjustedby adjusting the relative magnitudes of the energizing currents suppliedto the two windings. In a preferred adjustment, the flux lines underconsideration extend conelike between the two windings at an angle ofabout 45 to the horizontal.

An annular electron gun directs a conelike electron beam downwardly andinwardly along the aforesaid conelike, converging magnetic flux lines.The magnetic field guides the beam into the open top end of the castingmold and maintains the desired beam pattern under adverse conditions,such as the evolution of large quantities of gaseous matter whichbecomes ionized and forms highly conductive plasmas in regions traversedby the beam. Preferably, melt stock is introduced downward through thehollow, conelike electron beam, and is melted off from a preciselylocated melting surface a small distance above the open top end of thecasting mold.

The foregoing and other aspects of the invention may be understoodbetter from the following illustrative description and the accompanyingdrawings.

FIG. 1 of the drawings is a highly schematic, vertical section of animproved electron-beam furnace.

FIG. 2 is a fragmentary schematic, vertical section of the same furnace,drawn to a somewhat larger scale, showing typical magnetic flux linesand electron trajectories.

Referring to the drawings, an annular, copper mold 1, with its axivertical, has open upper and lower ends and is provided with a waterjacket 2 through which water or other coolant is continuously circulatedby conventional means (not shown), whereby the mold is cooled tosolidify molten material therein. Other parts of the furnace may also bewater-cooled, as desired, such being accomplished by obvious meansrequiring no description. The solidified material may be progressivelywithdrawn through the open bottom end of mold 1 to form a cast ingot 3of progressively increasing length, which may be cut off from time totime as desired. Progressive withdrawal of the ingot is accomplished,for example, by means of rollers 4 driven by an electric motor 5.

A first annular electromagnet winding 6 extends coaxially around mold 1,as shown, and has a vertical axis concentric with the open top end ofthe mold. Preferably, this winding is protected by an inner sheath 7 ofinsulation and an outer sheath 8 of metal. Wires 9 and 1d connectwinding 6 to a DC. power supply 11 in series with a rheo-stat 12,whereby the winding 6 is supplied with energizing direct current ofadjustable magnitude. A second electromagnet winding 13 is verticallyspaced above the first winding, in vertical, coaxial alignment with thefirst winding '6 and the annular mold 1. Preferably, winding 13 isprotected by an inner sheath 14 of insulation and an outer sheath 15 ofmetal. Wires 16 and 17 connect winding 13 to D.C. power supply 11'(which in practice may be combined with supply 11, if desired) in serieswith a rheostat 18, whereby winding 13 is supplied with energizingdirect current of adjustable magnitude.

The two windings 6 and 13 are energized in bucking magnetic fluxrelation and the energizing currents, supplied to the two windings areindividually adjustable by means of rheostats 12 and 18. Theampere-turns energization of winding 6 is usually greater than that ofwinding 13, typically about two times greater. The so-energized windingsproduce a magnetic field having magnetic fiux lines that extend inwardlyand downwardly between the two vertically spaced windings. A majority ofthese flux lines converge downwardly through the lower, more stronglyenergized winding 6, as represented 'by broken lines 19, FIG. 2. Aminority of the flux lines turn upward through the upper, less stronglyenergized winding 13, as represented by broken lines 20.

The flux lines of chief interest are lines 19, and particularly those oflines 19 that converge into the open top end of casting mold 1. Thestrength of the bucking field established by winding 13 sets the angleat which lines 19 pass between the two windings, e.g., as the buckingfield is made stronger, the flux lines are pushed downward, theZeno-field surface separating lines 19 from lines 20 being a cone thatapproaches a horizontal plane as the ampereturns energization of winding13 approaches that of winding 6.

An annular electron gun directs a beam of electrons along the magneticflux lines 19 that converge into the open top end of casting mold 1. Inits preferred form, the electron gun comprises an annular, thermioniccathode 21, most commonly made from a horizontal loop of tungsten wireconnection through leads 22 and 23 and a transformer 24 to analternating-current supply 25, which supplies alternating currentthrough wire 21 for heating the same to thermionic-emission temperature.An accelerating electrode 26 is closely spaced inwardly from cathode 21,and a focusing electrode 27 is closely spaced outwardly from cathode 21,as shown. Electrical connections are provided for maintainingaccelerating electrode 26 at substantially the same electric potentialas mold 1, preferably ground potential. This is indicated schematicallyin the drawings by ground connection symbols 28 and 29. Cathode 21 andfocusing electrode 27 are maintained at substantial negative potentials,commonly 5,000 to 15,000 volts, relative to the accelerating electrode.This is accomplished, for example, by means of connection 30 betweenelectrode 26 and lead 23 and by connection 31 between lead 23 and thenegative terminal of a high-voltage D.C. supply 32.

The overall design of the electron gun may be similar to that describedin the copending patent application of Charles W. Hanks, Serial No.818,306, filed June 5, 1959, and assigned to the same assignee as thepresent application. In the present furnace employing a magnetic fieldto guide the electron beam, the cathode, the accelerating electrode, andthe focusing electrode are shaped and aligned to direct electronsdownwardly and inwardly parallel to the converging magnetic lines offorce 19, forming a hollow, conelike electron beam, and thereafter themagnetic field plays a significant part in focusing and guiding theelectron beam into the open top end of mold 1.

A vertical rod 33, of material that is to be melted, is supported abovemold 1 in vertical, coaxial alignment therewith, and is continually feddownwardly into the hollow, conelinke electron beam. Feed mechanism issymoblized by rollers 34 driven by an electric motor 35. The bar of meltstock is electrically grounded, e.g., through rollers 34 as symbolizedby the conventional grounding symbol at 36. As the lower end of rod 33advances into the electron beam, the melt stock is melted away and theso-melted material falls into the open top end of mold 1 for continuallyreplenishing a pool 3' of molten material, which rests on top of castingot 3 and is supported within a skull or depression that formsautomatically atop the solidifying material.

An important function of the improved magnetic beam guidance which thisinvention provides is to prevent the electron beam from climbing up therod 33 of melt stock, or otherwise being diverted away from the pool 3'.Heretofore, this has been a problem which limited the size of ingotsthat could be processed, particularly with vertically fed melt stock, itbeing increasingly difficult to project the beam inward between the rodof melt stock and the molten pool, while preventing substantialdiversion of the beam to the melt stock and maintaining uniform heatingof the whole surface of the pool, as ingot diameters are increased toobtain larger-scale and more economical operations. The problem is notonly that the beam must travel inward farther between the two heatedsurfaces, but also, with the larger ingots greater power, and thereforemore beam current, is required, and larger quantities of gaseous matterare evolved from the melting and molten material.

The present invention has been found eifective in overcoming the abovedifliculties, and apparently overcomes all the limitation heretoforeexisting on maximum ingot size. The inside edge of the magneticallyguided, conelike beam remains sharp and well defined, whereby the meltstock is melted away at a precisely located, conelike surface. Hence,the melting rate is easily controlled merely by controlling the rate atwhich the melt stock is fed into the beam and the beam remains wellfocused over the entire surface of the molten pool. Further, thediameter of the electron gun can be increased greatly, which not onlyremoves the gun from regions containing relatively high concentrationsof evolved gaseous matter, but also increases the electron-emitting areaof the gun so that a greater total beam current is obtained in relationto the electronic current intensity at any given point near theelectrodes of the gun.

In FIG. 2, the volume occupied by the conelike electron beam isrepresented by the shading between lines 37 and 38 and between lines 39and 40. It will be noted that the beam is everywhere substantiallyparallel to the magnetic flux lines 19 which converge into the open topend of mold 1 and guide the beam.

In FIG. 1, there is schematically shown a vacuum tank at which enclosesthe casting mold, the electron gun, and associated parts. Tank 41 iscontinuously evacuated to a high vacuum, preferably one micron ofmercury absolute pressure or less, by connection of the tank through alarge-area duct 42 to high-capacity vacuum pumps 43. Appropriate airlocks (not shown) may be provided as desired for the introduction ofmelt stock, the removal of ingots, the replacement of electron guns, andthe like.

It will be understood that the specific embodiment illustrated is butone example of how this invention may be carried out, and that numerouschanges and modifications are possible without departing from theinventive principles herein disclosed.

What is claimed is:

1. An electron-beam furnace comprising a container for molten material,said container having an open top, two vertically spaced, coaxialelectromagnet windings, one of said windings extending around saidcontainer and the other being vertically spaced above the open top ofsaid container, means for energizing said windings in bucking magneticflux relation, the so-energized windings providing a magnetic fieldhaving converging flux lines extending between said vertically spacedwindings and into the open top of said container, and electron gunaligned to project a beam of electrons in a direction extendinglaterally with respect to the common axis of the windings and along saidconverging flux lines between said windings into said container, avacuum tank enclosing at least the space between said gun and saidcontainer, and means for evacuating said tank continuously.

2. An electron-beam furnace comprising an annular container for moltenmaterial, said container having an open top end, two vertically spaced,annular electromagnet windings disposed in vertical, coaxial alignment,one of said windings extending coaxially around said container and theother being vertically spaced above the open top end of the container,direct-current supply means connected to energize said windings inbucking magnetic flux relation, the ampere-turns energization of thelower winding exceeding the ampere-turns energization of the upperwinding, the so-energized windings providing a magnetic field havingconverging flux lines extending downwardly and inwardly between the twovertically spaced windings into the open top end of said container, anelectron gun aligned to project a beam of electrons along saidconverging flux lines between said windings into said container, meansfor feeding material to be melted downward through the upper annularwinding into said beam, whereby the fed-in material is melted by theelectron beam and falls into said container, a vacuum tank enclosingsaid container and gun and the space therebetween, and means forevacuating said tank continuously.

3. An electron-beam furnace as in claim 2, said electron gun comprisingan annular, thermionic cathode in vertical, coaxial alignment with saidtwo windings and container, an accelerating electrode closely spacedinwardly from said annular cathode, a focusing electrode spacedoutwardly from said cathode, means maintaining said acceleratingelectrode and container at substantially equal electric potentials, andmeans maintaining said cathode and focusing electrode at substantialnegative potentials relative to the accelerating electrode, saidfocusing electrode and cathode and accelerating electrode being spacedand aligned to form a hollow, conelike electron beam directed downwardlyand converging inwardly, being everywhere substantially parallel to saidconverging magnetic flux lines.

4. An electron-beam furnace for melting rods of material and castingingots therefrom in a high vacuum, said furnace comprising an annular,metal mold having a vertical axis and an open top end, means forcontinuously cooling said mold to solidify molten material therein, afirst annular electromagnet winding extending coaxially around said moldbelow said open top end, a second annular electromagnet winding spacedvertically above the open top end of said mold, said second windingbeing in vertical, coaxial alignment with said first winding and mold,direct-current supply means connected to energize said first and secondwindings in bucking magnetic flux relation, the ampere-turnsenergization of said first winding exceeding the ampere-turnsenergization of said second winding, the so-energized windings providinga magnetic field having converging flux lines extending downwardly andinwardly between the two vertically spaced windings into the open topend of said mold, means for individually adjusting the energizingcurrents of said windings, an annular electron gun coaxial with saidwindings and vertically positioned to direct electrons between the twovertically spaced windings, said gun comprising a thermionic cathodeconsisting of a horizontal loop of wire and connections for supplyingheating current therethrough, an accelerating electrode closely spacedinwardly from said cathode, a focusing electrode spaced outwardly fromsaid cathode, means for maintaining said accelerating electrode atsubstantially the same electric potential as said mold, and means formaintaining said cathode and focusing electrode as substantial negativepotentials relative to the accelerating electrode, said focusingelectrode and cathode and accelerating electrode being shaped andaligned to form a hollow, conelike electron beam directed downwardly andconverging inwardly, being everywhere substantially parallel to saidconverging magnetic flux lines extending between said windings into theopen top end of the mold, a vacuum tank enclosing said electron gun andsaid mold and the space therebetween, means for continuously evacuatingsaid tank to a high vacuum, means for feeding rods of melt stockdownward endwise through said second annular winding toward the open topend of said mold in vertical alignment therewith and into the hollow,conelike electron beam, and means for maintaining the rod of melt stockat substantially the same electric potential as said mold, whereby thelower end of the rod of melt stock is melted away by the electron beamand the so-melted material falls into the open top end of the mold.

References Cited in the file of this patent UNITED STATES PATENTS2,321,886 Anderson June 15, 1943 2,423,729 Ruhle July 8, 1947 2,880,483Hanks et a1. Apr. 7, 1959

1. AN ELECTRON-BEAM FURNACE COMPRISING A CONTAINER FOR MOLTEN MATERIAL,SAID CONTAINER HAVING AN OPEN TOP, TWO VERTICALLY SPACED, COAXIALELECTROMAGNET WINDINGS, ONE OF SAID WINDINGS EXTENDING AROUND SAIDCONTAINER AND THE OTHER BEING VERTICALLY SPACED ABOVE THE OPEN TOP OFSAID CONTAINER, MEANS FOR ENERGIZING SAID WINDINGS IN BUCKING MAGNETICFLUX RELATION, THE SO-ENERGIZED WINDINGS PROVIDING A MAGNETIC FIELDHAVING CONVERGING FLUX LINES EXTENDING BETWEEN SAID VERTICALLY SPACEDWINDINGS AND INTO THE OPEN TOP OF SAID CONTAINER, AND ELECTRON GUNALIGNED TO PROJECT A BEAM OF ELECTRONS IN A DIRECTION EXTENDINGLATERALLY WITH RESPECT TO THE COMMON AXIS OF THE WINDINGS AND ALONG SAIDCONVERGING FLUX LINES BETWEEN SAID WINDINGS INTO SAID CONTAINER, AVACUUM TANK ENCLOSING AT LEAST THE SPACE BETWEEN SAID GUN AND SAIDCONTAINER, AND MEANS FOR EVACUATING SAID TANK CONTINUOUSLY.